1. Field of the Invention
This invention relates to an apparatus and process for aluminothermic welding without preheating of metal parts, with rising channels controlling such process.
2. Prior Art
In aluminothermic welding without preheating of two or more identical or similar metal parts of variable cross-sections, such as rails, aligned in an end-to-end relation, several methods are known in which the amount of heat required to weld the rail ends is supplied solely by the liquid steel alumino-thermically produced.
In one such process and apparatus, described in U.S. Pat. No. 3,189,959, the rail ends are aligned with a gap therebetween and enclosed in a welding mold, which contains rising channels and ventilating ducts. The casting crucible is positioned above the mold and is manually tapped when the aluminothermic reaction is completed. The pouring liquid steel is divided into two stream halves which join in the region of the center of rail web and then fill the welding cavity and the rising channels.
It has been found, however, that the method is subject to several disadvantages. The liquid steel is divided into two stream halves, which are in direct contact with the mold, which is cold at the beginning of welding process. Consequently, the liquid steel loses a part of its heat before it reaches the rail ends. Then, the first portion of liquid steel, which is poured into the welding cavity in the early stage, gives the greatest amount of heat to the rail in the region of center of rail web, where it first reaches the rail ends. As a result, the rail web, which is the thinnest part of the rail, is melted to a greater extent compared to the other portions of rail, and also, the first portion of the liquid steel which reaches the rail bases, and the rail base extremities in particular, could not have the required temperature to properly melt the rail in the region of base extremities. The subsequent liquid steel which is poured into the welding cavity gives the greater amount of heat to the rail in the region of the surface of the rising liquid bath in the welding cavity, and thus it would not have a significant effect in melting the rail bases.
The area of the cross-section of rising channels is practically constant, within known constraints of existing manufacturing technology. Consequently, the rising channels do not control the amount of heat which is given by the liquid steel to the rail in the region of the surface of the rising liquid bath. Thus, the rail is not uniformly melted on the entirety of its cross-section, resulting in internal stresses after the weld is cooled. In particular, the rail web is melted to a greater extent compared to the rail head, and a retreat in the weld in the region between the rail head and the rail web would occur. The rising channels are located above the extremities of rail bases with the result that the liquid bath rises rapidly in the region of rail bases and the rail bases are not properly melted. Further, if the method is used to weld rail sections in an elevated track, for instance in curves, and the center line of mold deviates from the vertical, then the lower base receives a greater amount of liquid steel, with the result that the elevated base would be insufficiently melted.
In the process and apparatus, described in U.S. Pat. No. 4,250,944, the welding operation is performed in a similar manner as in the previous method. However, the two stream halves join along the longitudinal axis of rail, toward the two rail heads. This method is also subject to certain disadvantages. Due to the increased height of the mold, the length of path along which the liquid steel flows from the outlet of the crucible to the extremities of rail bases is inevitably increased, and the liquid steel loses heat until it reaches the rail ends. The liquid steel further loses heat because of the direct impingement realised by the two stream halves onto the center of rail heads. Consequently, the liquid steel loses a significant amount of heat until it reaches the rail bases and the rail bases are not properly melted, resulting in an uncertain weld, especially in the case of a gap of minimum size.
In another process and apparatus, described in U.S. Pat. No. 4,605,053, the rail sections are aligned and enclosed in a welding unit, consisting of a welding mold and a reaction crucible, separated by control disks. The welding mold contains a mold cavity, discharge chambers located underneath the rail base, recesses, exit gates and ventilating ducts. The reaction crucible is located in the upper side of the welding unit, above the gap, between the two rail ends and is automatically tapped when the control disks are melted by the liquid steel. Then the liquid steel fills the mold cavity and the discharge chambers, realising at the same time a direct impingement onto the top of the rail bases.
However, this method is subject to several disadvantages. Given the geometry of the center of the welding cavity and of the recess which is in the wall of mold, the frictional surface of the former is much smaller than the frictional surface of the latter. Also, the wall of the recess inclined toward the center of mold cavity has the effect of a greater speed of the liquid through the center of the welding cavity. Consequently, in the first stage of casting, the liquid flows at a greater speed through the center of the welding mold. Thus, the mold cavity would be completely filled practically with the speed at which the liquid is poured from the reaction crucible, and the first pouring phase, which is the period of time during which the first portion of liquid steel flows along the rail bases and the liquid bath has not started to rise in the welding cavity, would be extremely short. Consequently, the rail bases would not be properly melted in the first stage of pouring, and the subsequent liquid steel poured into the welding cavity would not have a significant effect in melting the rail bases. After the welding cavity has been filled, the liquid flows with a lower speed, and thus it gives the biggest amount of heat to the rail heads. The recess does not apparently bring any benefit to the welding process, but the two pairs of double ribs which inevitably result from the recess provide a major disadvantage in the case of a broken weld, since it is extremely difficult to secure the rail with cover plates until it is properly repaired.
The aluminothermic reaction depends on the composition of the welding material, the extent at which it is dried, the age of the welding material, and other factors. Once the reaction is started, the liquid steel cannot be prevented from pouring into the welding cavity, and in the case of an abnormal progress of the reaction, this would result in a defective weld which has to be cut away, which is time-consuming and expensive. The welding cavity is not provided with means for ventilating the gases which develops therein during the pouring of liquid steel and this would cause pores in the weld. Also, serious difficulties are encountered when this method is used to weld wet rails, as it is the case in cool mornings, because the welding unit is not provided with proper means by which a burner could effectively reach and dry the rail. The burner is a bulky device, which is difficult to fabricate, to handle and to transport, resulting in an expensive mold.
In another process and apparatus, described in U.S. Pat. No. 3,620,291, the rail ends are aligned and enclosed in a welding mold, which contains rising channels, stoppers and ventilating ducts. After the casting crucible is manually tapped, the pouring liquid steel is divided into two stream halves which join in the region of the rail base and then fill the welding cavity and the rising channels.
However, this method is subject to several disadvantages. Practically, the casting operation has two phases. In the former, the discharge channels are closed by means of stoppers, and the welding cavity is filled with liquid steel. The latter phase starts when the stoppers are melted, then the level of the liquid bath decreases until an equilibrium of liquids is obtained between the welding cavity and the rising channels, and finally the welding cavity is filled with liquid steel, at a lower speed than in the first phase. Because of the geometry of welding cavity in the region of rail web and because the area of cross-sections of the rising channels is constant, the rail web receives the biggest amount of heat. The result is that a retreat would appear in the weld in the region between the rail web and the rail head. The liquid bath starts to rise in the welding cavity from the begining of the casting operation, with the result that the rail bases would be insufficiently melted Also, after the welding cavity is filled in the region of rail bases, the liquid flows to the exit gates at the level of the upper side of rail bases, as the exit gates are located at the upper side of the rail bases. Thus, subsequent liquid steel poured into the welding cavity would not have a significant effect in melting the rail bases. Furthermore, if the center line of the mold deviates from the vertical, as it is the case in an elevated track, the elevated base would be insufficiently melted.
In another process and apparatus, described in Romania Pat. No. 80,214, the rail ends are positioned with a gap therebetween and enclosed in a welding mold, which contains stepped rising channels, connecting gates and ventilating ducts. The area of the cross-section of a rising channel varies along said channel in such a manner that areas in the longitudinal section of different portions of a channel are proportional to areas of corresponding portions of rail. After a casting crucible is manually tapped, a single liquid stream flows into the welding cavity centrally through the gap and fills the welding cavity and the rising channels.
However, this method is subject to several disadvantages. The connection from the welding cavity to a rising channel is employed by a pair of connecting ducts located in the region of rail base, more specifically, at the upper side of base portion of the welding cavity. Consequently, the cooled liquid steel cannot be entirely eliminated from the lower side of welding cavity in the region of the extremities of rail base, especially when rails of large size are to be welded. Because the two cores used in employing the rising channels are in an inter-dependent relation, the rising channels are not sufficiently large in the region of rail head, and the rail head would not be properly melted. The rising channels are significantly conic along their heights to allow the cores to effectively be positioned into the rising channels, with the result that the volume of the lower side of rising channels would not be sufficient to properly melt the rail bases. Therefore, the mold requires large rising channels, which in turn require an increased longitudinal dimension of the mold. This prevents it from being used in performing welds in narrow spaces, which is the case of high-speed switches and of switches having a single-piece heart. Finally, technical difficulties are encountered when the stepped rising channels are to be employed with inter-dependent cores.